Network Architecture.

Lecture #2- Computer Networks.Types and Applications.

Network Architecture.

C o n t e n t s

w Introduction

w Application of the Computer Networks

w Network Architecture - hardware and software

w Network Types

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Introduction - Terminology

Computer Network:

interconnected autonomous computers

explicit addressing and naming virtual

explicit allocation / reallocation memory

explicit remote management

Distributed System:

interconnected autonomous computers

transparent addressing virtual

transparent allocation and reallocation uniprocessor

transparent execution

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Application of the Computer Networks

w

Enterprise Networks (Intranets)

w

Public Networks

w

Personal Use

w

Social Aspects

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Enterprise Networks (“Intranets ”)

w Resource Sharing - many computers in different places

w High Reliability - duplication of data, hardware resources, fault-tolerance

w Low Cost/Performance Ratio - cheaper

workstations than mainframes; application of the client-server model

w Scalability and Flexibility - system grows with the enterprise

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Public Networks

w 2 preconditions: cheap and compact home computers and communication technologies

w Electronic commercial and banking,

entertainment, public and social services, mass media etc.

w Person-to-person communication

w Instances: WWW, E-mail, electronic

newspapers, on-line TV/Radio, newsgroups, videoconferencing etc.

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Personal Use

w Moving all the services at home - shopping, banking, health, TV/radio/cinema/news-

papers etc.

w On-line information - “Browsing”,

“Surfing”, searching machines.

w Personal contacts and communication.

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Social Issues

w Formal (Legislative) Regulation of service providing (incl. taxes, QoS/protection of customer rights, registration of

sites/addressing etc.)

w Electronic Payments and Security Issues

w Advertisement (incl. Internet,

“spaming”/“bombing”)

w Information Correctness

w Public control (illegal sex, trade, crimes)

Clinton’s Communication Decency Act - 1996

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Network Hardware

w Main Taxonomy Dimensions:

F

F

w Transmission Technology

F

F

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Broadcast Networks

w Single communication channel shared by all the machines in the network

w Short messages (packets) with addressing field

w Target machine interprets current message;

the rest ignore it

w Special bits in the address field indicate the transmission mode: broadcast (to all the

machines); multicast (to group of machines)

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Point-to-point Networks

w Many connections between pairs of machines

w Packets visit 0, 1, 2 … intermediate machines reaching the target one

w Alternative routes - routing algorithms

w Basically for large networks

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Network Range

w Physical Size

w Data Flow Machines & Multicomputers ^are

Distributed Systems but not Computer Networks

w Local Area Networks (LANs)

w Metropolitan Area Networks

w Wide Area Networks (WANs)

w Internetworks (connection of more than one network - The Internet)

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fine grain parallel com- puters; many functional units

perform any instruction

message passing sys- tems; short and fast com-

munication busses

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Local Area Networks (LANs)

w Up to 1-2 km physical range (room, building, campus)

w Private owned (companies, branches, laboratories, small institutions)

w Connect PCs, workstations, disk stores, printers and other peripherals

w Main characteristics:

F

F

F

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Size of the LANs

w Small, technologically restricted size

w Bounded and known transmission time

w Simple network management due to the limited transmission time

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Transmission Technology of the LANs

w Usually based on a single cable attaching all the components of the network

w Communication speed 10 - 100 Mb/S

w Communication delay 10 - 100 µS (1 µs = 1^-6 S)

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Topology of the LANs

w Topology is the graph of the connectivity of the network components

w Typical topology of LANs is bus or ring

w Bus topology is based on linear cable

w Arbitration management - time-sharing control:

1 master machine at any instant is allowed to transmit; conflict requests resolvation

w Centralized or Distributed arbitration

w Ethernet^TM (IEEE 802.3) standard: bus-based, distributed arbitration based on collision

detection and random delay for next attempt

1/3

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Topology of the LANs - ring topology

w Network ring forms necklace of the workstations

w Bit-slice propagation of the packets

w IBM Token Ring (IEEE 802.5) standard operates at 4-16 Mb/S

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Local area networks

• Local area networks (LANs) are physically relatively small. They are usually spanning one km or less.

• Usually, every device on the LAN essentially shares the same transmission cable. This is called shared media

access.

• Ethernet LAN technology (IEEE 802.3) has today the

largest installed base. It has several types of cabling - Thick coaxial (10-base-5) with AUI connectors and transceivers, thin coaxial (10-base-2) with BNC connectors, and

unshielded twisted pair (UTP or 10-base-T) with RJ-45 jacks.

• Each station on an Ethernet has an Ethernet network

interface card (NIC), which has a special hardware address, assigned to guarantee link layer uniqueness, even across

vendors.

• Ethernet is mostly used with speeds 10 Mbps and 100 Mbps.

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Local area networks

• Ethernet media access control allows any station on the network try to transmit at any time, which may produce collisions when several stations transmit at one time.

• Media access mechanism identifies collisions and allows each station to have adequate access to common channel.

• In Ethernet this mechanism is called carrier sense multiple access with collision detection (CSMA/CD).

• Another quite a popular LAN technology is the Token Ring specified in IEEE 802.5 with basic speed of 4 Mbps.

• The media access in Token Ring is based on token passing mechanism.

• Other IEEE 802.x standards are 802.4 Token Bus (5 and 10 Mbps) used in Manufacturing Automation Protocol (MAP) systems, and 802.6 Metropolitan Area Network (MAN),

which is also called DQDP (Distributed Queue Dual Bus) and has speed 100 Mbps.

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Metropolitan Area Networks

w Bigger version of LANs using similar

technology - common broadcast media that connects all the computers

w Options: voice and image communications (incl. cable TV)

w No switching elements; 1 - 2 cables

w DQDB (Distributed Queue Dual Bus - IEEE 802.6) Standard: 2 unidirectional buses with 2 head-ends (for each bus) to initiate the

transmission activity in each direction. 1/4

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Wide Area Networks (WANs)

w WAN covers large geographic area (country, continent)

w Connects different types of machines -

“hosts” via communication subnet

w Separation of the services: hosts run

application programs and subnet performs the connection tasks

w Subnet consists of transmission lines and switching elements (“routers”: specialized in connecting 2 or more switching lines)

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WANs & Routers

w Router is specialized switching element in the WAN subnet.

w Switching is the process of:

1) receiving data on the incoming channel[s];

2) interpreting it;

3) choosing an outgoing line and 4) forwarding the data on it.

w Typical structure of WAN: hosts connected by LANs; LANs connected by the subnet

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WANs & Subnets

w Subnet consists of routers and connection lines

w The subnet lines are based on cables

(telephone lines) that connects pairs of routers (point-to-point network) in a connected graph.

Exception: wireless/satellite based subnets

are of broadcast type (for WANs specialized in broadcasting communication)

w Non connected routers communicate via intermediate routers in store-and-forward mode

w Subnet topology - usually irregular 1/6

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Wide area networks

w Wide area network (WAN) is a concept

that is used in context of geographically large networks (1 - 50 km span).

w Generally, WANs consist of site specific LANs and teleoperator backbones which are used to interconnect the LANs.

w This trunk capacity is provided to WANs as:

• Basic analogue telephone connection with modems

• Digital ISDN connection with terminal adapters (TA)

• Connection with direct router support or using Frame Relay (FR)

• SDH based connection with ATM.

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Wide area networks

w Basic telephone networks support switched 64 kbps connections with either analogue modem or digital ISDN subscriber lines.

w ISDN Basic Rate Interface (BRI) consists of two bearer (B) channels and a 16 Kbps signaling (D) channel (called as 2 B + D).

w ISDN Primary Rate Interface (PRI) is a high-

bandwidth version of the BRI, also called as 30 B + D.

w Serial Line Internet Protocol (SLIP) and Point-to- Point Protocol

w (PPP) are used with the basic analogue and digital telephone interfaces as the transport layer for an

Internet link.

w SLIP/PPP connections are temporary links over standard serial phone links between end user and terminal server.

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Wireless Networks

w Preconditions: mobile computers + digital wireless communications

w Application: portable office, transportation business, emergency services, police/

military, etc.

w Features:

C easy installation, portability

D low capacity (1-2 Mb/S), low security, high error rate + radio pollution

w Wired-Wireless Networking Convergence

w Tendencies

Laptops, PDAs binary coded radio transmission or CDPD (Cellular Digital Packet Data)

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Internetworks

w Internetwork = Communication between LANs, MANs and WANs with different internal standards

w Compatibility requires gateways

w Typical architecture: collection of LANs connected by a WAN: (WAN

differs from the subnet just by presence of hosts besides the routers)

w The Internet = biggest internetwork

connecting universities, public and private offices, persons etc.

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1/5

1/01/0

Communication system:

• parallel:

DMA to remote shared memory standard common bus

...

•

• serial::

modem connection standard LAN ...

Architecture of a Distributed System / Network of computers

Network Software:

Characteristics

w Based on structural programming approach

w Network Layers: hierarchy of SW modules providing communication services to the next upper layer

w Transparency of the layered structure:

independence of layer n of the

implementation of the lower layers

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Network Software: Structure

w Layered structure:

üProtocol - rules and convention of data exchange between layer n of host1 and layer n of host2

üPeers - entities that locally implement the functionality of a given layer

üInterface - the set of primitive operations and services that lower layer provide to the upper one

üPhysical media - the signal carrier that is used by the 1^st layer for transmission

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Network Software

w Considerations:

üVirtual exchange between the equilevel layers of two hosts according the

protocols

üPhysical exchange between the neighbor layers of one host according to the

interface

üPortability of the layers: based on clear simple interfaces and well defined set of functions of each layer

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Network Architecture

w Network architecture - the set of layers and protocols; ignores the interfaces as the

interfaces of the hosts in a network may differ

w Protocol stack - the list of protocol

hierarchy in the network; matches the layered structure

w Analogies to the network protocol stack

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1/10

Network Architecture

w Example network architecture:

ÿ5-layer protocol stack

êLayer 5: Application process generates message M and deposit it to Layer 4

êLayer 4: Extends M with the header H₄

containing control information (ordering, size, time, etc.) and deposits H₄M to Layer 3

êLayer 3: Brakes H₄M into smaller fixed size

packets (e.g. H₄M₁ and M₂); extends them with its header H₃; selects an outgoing line for

transmission and passes the packets to Layer 2 êLayer 2: Adds its header and trailer to each

packet and deposit them to Layer 1 for physical transmission

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Network Architecture

w Receiving of the message M at the destination machine consists in:

• moving of its packets upward the layers,

• stripping of control header and trailers,

• merging the packets in a message and

• interpreting the message by the application

w Lower layers have hardware implementation

w Medium layer[s] have firmware implementation

w High layers have software implementation

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Layers Design

w Layer’s design issues:

~identification mechanism for senders/receivers - process ID, machine ID, net ID, etc.;

~data transfer mode - simplex, half-duplex and full-duplex

~support of multiple logical channels with priority scale

~Application of error-detecting and error-

correcting codes and mechanisms for feed-back

~ordering protocols for the packets in messages

~buffering between fast and slow processes

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Connection-Oriented and Connectionless Services

w Connection-oriented service: establishes the connection from point to point; caries the exchange, preserving the order of the

bitstream and releases the connection.

Analogy to telephone system.

w Connectionless service: each message is

provided with full destination address and it is routed through the system independently to rest of message stream.

w QoS (quality of service) - reliability to losing data

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Quality of Services (QoS)

w Implementation of reliability: based on acknowledgment by the receiver -

acknowledge receipt for each message

w Acknowledge receipts produce

• communication overhead and

• delays

w Application: file transfer

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Unreliable Connection-Oriented Service

w Application for systems where delays are unacceptable, e.g. real-time systems for

w voice communication

w on-line image transmission

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Unreliable Connectionless Service

w Application - all functions where:

• real-time, interactive or on-line features are not essential but

• the cost of communications has to be minimized and also

• reliability is not of crucial importance w Example: standard e-mail services

w Implementation: datagrams - not

acknowledged connectionless service

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Reliable Connectionless Service

w Application: non-interactive short messages exchange with guaranteed reliability

w Example: banking, military, remote queries in data bases

w Implementation: acknowledged datagrams

w Variation: Request-Replay services for one- cycle interaction. Mostly in remote data-

base access and another client-server applications

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Service Primitives

w Set of operations - primitives - forms the access language to a service. Primitives:

3

* inform the service process for some event in the peer entity

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Confirmed and Unconfirmed Services

w Confirmed Services: the exchange of primitives between the peer entities follows the pattern:

)

Ï

Ñ

Ð

w Unconfirmed Services: the exchange of

primitives between the peer entities follows the pattern:

)

(

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Application: basically for

(connection oriented services

‘

Application: basically for

* connectionless services

. unreliable services

Reference Models - Basics

w Reference models, ISO

w OSI = open systems interconnectionopen systems

w Layers:

N Perform similar functions N Process similar data

N Respect internationally standardized protocols N Minimize the information flow though the

interfaces

N Their number is the smallest possible to mach all different levels of protocol abstraction

• Examples: ISO 7 layers; internet 5 layers

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Reference Models - the OSI Model

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w The OSI model:

F

F

F

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Protocol stacks

w OSI protocol stack

OSI-protocols are specified in seven layers. The lower layers are more hardware and transmission oriented. The upper layers are oriented to presentation and

synchronization purposes. The middle layers handle network quality, addressing and routing.

w Layers with example OSI protocols are:

Application Presentation Session Transport Network Data link Physical

FTAM, ACSE, ROSE OSI Presentation

OSI Session BSS, BSC, BAS OSI Transport Class 0,..,4 OSI Network, X.25

HDLC

Voltages as X.24 7

6 5 4 3 2 1

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Communication Functions according to the OSI Model

User applications ..

Encryption/

decryption

compression/

expansion

Choice of syntax Session

control

Session to transport mapping

Session management Session

synch.

Layer and flow control

Error recovery

Multiplexing

Connection control

Routing Addressing

Error control

Flow control Data link

establishment

Synch Framing

Access to transm. media

Physical and electrical interface

Activation/

deactivation of con.

Application layer Presentation layer

Session layer Transport layer Network layer

Link layer Physical layer

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The OSI Model - The Physical Layer

w

Bit-slice transmission via some communication channel e.g.

@ Method of bit coding 0/1

@ Physical parameters:

voltage/amperage etc.

@ Timing: frequency/period, shape of signal front, etc.

@ Direction[s] of transmission

@ Establishment and canceling of the connection

@ Physical/mechanical interfaces to the connection medium (e.g. RS234

connector)

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The OSI Model - The Data Link Layer

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Maintains the error-free error-free transmission line line for data frames serving the data frames

requests of the upper Network Layer.This includes:

braking the upper level data into or

packing the lower level bit stream into frames

frames

keeping the data sequencesequence by exchange of acknowledgement frames

create or recognize frame boundaries by bit patterns for beginning/end frame boundaries

The OSI Model - The Data Link Layer

š retransmission of corrupted or erroneous frames

š manages problems of duplicate,

corrupted or lost frames depending on the service (price/speed) level applied by the upper layers

š low level buffering between upper layers peers of different capacity

š support of bi-directional communication:

incoming data frames share the line with outgoing acknowledgement frames

š for broadcast networks: medium access sublayer for shared channel control

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The OSI Model - The Network Layer

ww Subnet control layer i.e. Subnet routing of of therouting Data Link Layer packets from source to destination. Routing might be:

P P

— —

j j

w Routing trends to solve problems with temporarily bottlenecks

w Network layer also does the following:

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The OSI Model - The Network Layer

ä

customer/network etc.

ä

ä

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The OSI Model - The Transport Layer

w Exchange (“transport”) of data “point-to-point-to- point

point” providing the upper (session) layer with error-free data messages. It cares for:error-free

ý effective communication - for high throughput it might open >1 network connections -

“multiplexing”multiplexing ý fault tolerance

ý opening/closing the connections with named parties in the network + support of naming mechanism needed - “flow controlflow control”

ý different types of services: point-to-point channel; isolated messages; broadcasting.

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The OSI Model - The Session Layer

w Establishes sessions between network

machines. The sessions are extensions over the transport layer communication, that

support:

:

^ interactive exchange (dialogue):

C bi-directional simultaneous È bi-directional alternative º uni-directional

^ dialogue synchronization - by session brakes

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The OSI Model - The Presentation Layer

w Interprets the exchanged data as information considering its syntax and semantics. This includes:

w security coding/decoding

w presenting data as text stringstext strings, formattedformatted numbers

numbers (integers, fixed, floating, double, etc.) according different formatting codes in both directions:

• local computer standard

• network standard

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The OSI Model - The Application Layer

w Set of protocols providing network-wide compatibility of the user programs

including:

7 full-screen terminal compatibility

> file- and directory- structure compatibility

‰

- electronic mail

………….

w Solution: network virtual standard to which to translate local structures/objects

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The OSI Model - example Data Transmission

w Sender transmits Data to Receiver

w The protocols implementing each OSI layer add special header to the Data (header

might be null)

w The lower level deals with extended Data (Data+Header) as a whole

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Reference Models - the TCP/IP Model

µ Developed for ARPANET (70ties US

national military network) and inherited in the Internet

J Features:

B

é

Ê

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1/18

Protocol stacks

TCP/IP stack

• Internet networks are based on TCP/IP protocols, so the TCP/IP model and protocol stack have a growing

importance.

• TCP/IP is based on five protocol layers instead of seven.

The OSI model session and presentation layers can be considered empty in TCP/IP context.

• TCP/IP stack with example protocols is shown below:

Application Transport Network Data link

Telnet, FTP, SMTP, SNMP, HTTP TCP, UDP

IP

HDLC or LAN frames Voltage levels

Physical

7 4 3 2 1

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TCP/IP Layered communication

Client Server

Router Telnet request

TCP segment IP datagram Ethernet frame

Voltage

Telnet request TCP segment

IP datagram Ethernet frame

Voltage IP datagram

Ethernet frame Voltage

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The TCP/IP Model - The

“Host/Network Layer”

w Corresponds to OSI Physical+Data Link Layers

w Unspecified strictly as protocol

w implementations vary in different networks and even hosts

w only restriction: serving upper (internet) layer in transmission of data packets

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The TCP/IP Model - The Internet Layer

w Connectionless layer (in order to provide the flexibility needed)

w Implementation: IPIP

w free independent exchange of packets (IP datagrams) transparently to the sender and receiver Üroutingrouting is a key issue in IP

w standard packet format (strictly supported)packet format for proper routing

w corresponds to OSI Network Layer

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The TCP/IP Model - The Transport Layer

w Supports “point-to-point” connectivity

between the source and destination (like OSI transport layer)

w Implemented by two protocols:

J J

connection oriented, delivers the byte stream from source to destination by fragmentation into discrete messages for transmission by IP.

Receiving TCP assembles the incoming messages to output stream

L L

connectionless, unreliable, non-sequential, for prompt delivery (multimedia applications)

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The TCP/IP Model - The Application Layer

w Top level protocols (session and

presentation layer functions are performed by the application when needed) like:

:

<

-

5

û

”

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1/19

Reference Models - OSI vs.

TCP/IP

w Similarities:

•• structure: stack of protocolsstructure

•• functionality: routing + point-to-pointfunctionality

connectivity + application supporting functions w Dissimilarities (OSI)/(TCP):

• conceptuality/applicability

• hidden, transparent, replaceable protocols / conservative, non-conceptual approach

• mostly connection oriented / pure connectionless oriented

• 7 layers / 4 layers

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Example Networks - The

ARPANET

w [Defense] Advance Research Project Agency - consists of subnet and hosts

w Subnet is based Interface Message Processors (IMP) connected by communication lines.

• Software: IMP/IMP- Host/IMP- and Host/Host- protocols

w Development - chiefly US universities: 1969, 70, 72, 73

w Extensions: Terminal Interface Processors (TIP) (Terminal Complexes), LANs, TCP/IP (protocol stack and model -1974), DNS (1981)

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Example Networks - The

ARPANET

[Defense] Advance Research Project Agency - first to adopt packet-switchingpacket-switching replacing

traditional circuit-switching. Advantages:

multiple routes rise fault-tolerance (dated) dense communication channels (actual)

Structure: subnetsubnet and hostshosts

Subnet structure: Interface Message ProcessorsInterface Message Processors (IMP) connected by communication lines;

Alternative connections for each IMP

Software: IMP/IMP- Host/IMP- and Host/Host-

protocols based on datagram exchange; rerouting algorithms for lost datagrams.

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1/24

Example Networks - The

ARPANET

w Development - chiefly US universities: 1969, 70, 72, 73

w Extensions:

• Terminal Interface Processors (TIP) (Terminal Complexes) - multiple host per TIP, multiplexed access of one host to several TIPs

• LANs

• TCP/IP (protocol stack and model -1974) suitable for mobile networks where a host can be switched to different networks of the subnet; since 1983 the only protocol stack of ARPANET

• DNS (1981) organization of host domains, namind all the hosts and mapping onto list of IP addresses

w Early 90’s ARPANET melted in arising Internet space

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Example Networks - The

Internet

The Internet arises on base of ARPANET after joining of another regional networks - NSFNET, BITNET, EARN, …, thousands of LANs; early 90’ the term “internet” widely accepted as net name “The Internet”

Internet machine is each machine that (1) inter- communicates with others under TCP/IP and (2) has a specific IP address

Classic applications: mail, news, remote login and file transfer

“New wave” applications: from gophers to WWW surfing

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Example Networks - Gigabit Implementations

w Next step after 10²Mb Internet backbones

w Specific Applications: Teleservices (on- line transmission of huge data arrays)

especially televideoservices, cable TV to net, etc.

G

bandwidth - for mass communications

w Implementations: mainly Ethernet LANs and ATM switches: 3Com^® (1000 megabits per second (Mbps) Gigabit Ethernet networking infrastructure around eleven 3Com CoreBuilder 9000 enterprise switches).

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